The invention relates to a heater with a ferro-electric ceramic heating element (PTC thermistor) having two heating surfaces which lie essentially opposite one another and which are both in heat-conducting connection with a member to be heated.
In the earlier U.S. Pat. No. 4,177,375 issues Dec. 4, 1979 of Hans Meixmer and the U.S. application Ser. No. 012,053 filed Feb. 13, 1979 of Hans Meixner, both of which are incorporated herein by reference, when using a disk-shaped ceramic heating element consisting, for example, of doped barium titanate, I have suggested to design the heat contact of this heating element relative to a member to be heated such that the heat produced by the two heating surfaces of the heating element is drawn off equally well from the two surfaces. An equal heat conduction on the two sides of the heating element is therefore preferred to an extremely good dissipation of heat conduction, for example, on one side only. This technique is used to guarantee the same thermal conditions and particularly the same temperature when the heating element is in operation, said element which advantageously is to be delimited to a thickness of only 0.5 to 2 mm across the thickness of the element. Therefore a self-stabilizing temperature of the heating element is guaranteed. The self-stabilization per se is based upon the PTC thermistor effect with a specific electric impedance rising suddenly with the Curie-temperature. The above reference provide details for design and construction of a PTC thermistor-ceramic heating element. However, difficulties can nevertheless sometimes occur in that a heat contact between the heating surfaces of the heating element, on the one hand, and the heat absorption surfaces of the member to be heated and positioned opposite thereof, on the other hand, exists. This heat contact is not yet even and sufficient.